1. Field of the Invention
The present invention relates to a method for manufacturing dielectric ceramic powder. More particularly, the present invention relates to a method for manufacturing dielectric ceramic powder by using wet-milled BaCO3 as raw powder to prepare raw ceramic powder via solid state reaction, thereby ensuring fine particle size and uniform particle size distribution, and a multilayer ceramic capacitor made from the ceramic powder.
2. Description of the Related Art
The information society of the 21st century has seen an increasing trend of digitalization, high-performance and high-reliability and multimedia in products such as electronic appliances, PC, HHP which chiefly utilize Multilayer Ceramic Capacitor (MLCC), one of the necessary passive devices of the electronics industry. Accordingly, MLCC parts have undergone higher-capacity and minimization fast. But this requires sheet lamination and fine BaTiO3 particles having uniform size distribution as the dielectric power used. Also, tetragonality indicated by a c/a ratio of c-axis to a-axis of BaTiO3 powder crystal needs to be higher (more than 1.008).
BaTiO3 powder has been produced by hydrothermal synthesis, alkoxide method, solid state reaction and the like.
In hydrothermal synthesis, jel-type titanium hydrate is added to a great quantity of barium hydroxide to react at a high temperature of about 150° C. and under a high atmospheric pressure of 10, thereby producing crystalline BaTiO3. This method has the advantage of directly producing spherical crystal BaTiO3 sized about 100 nm but has the disadvantage of difficult design and maintenance of a reactor, and expensive manufacturing cost. Furthermore, recently, BaTiO3 powder produced by hydrothermal synthesis has revealed significant defects such as oxygen vacancy and barium vacancy, which develop into pores in the case of heat treatment, thus deteriorating properties of BaTiO3 powder.
Meanwhile in synthesizing BaTiO3 via hydrolysis of metal alkoxide, metal alkoxide alcohol solution and Ba (OH) aqueous solution are mixed in a tube-type static mixer to react at a temperature of 80° C. This method is advantageous due to following reasons. Liquid phase employed herein as starting material is more reactive than solid jel-type titanium hydrate used in hydrothermal synthesis. This allows synthesizing at a relatively low temperature, and easy adjustment of the synthesized powder particle size to about 20 to 100 nm. However this method has drawbacks in that a synthesis device is hard to configure, and alkoxide reagent used as starting material is expensive. Furthermore, material cost is expensive due to use of alcohol solvent, and complicated process conditions such as synthesis temperature hamper mass productions.
Therefore, to manufacture low-priced BaTiO3, solid state reaction is most advantageous. In solid state reaction, BaCO3 powder and TiO2 powder are used as starting powder. The BaCO3 powder and TiO2 powder are mixed, and then undergo solid phase reaction in a calcination process to be synthesized into final BaTiO3 powder. To achieve lamination of dielectric layers, dielectric material powder should have small particle size and uniform particle distribution. But BaTiO3 manufactured by solid state reaction reportedly does not exhibit uniform particle size distribution compared to BTO manufactured via other methods described above. In the end, in sold state reaction, one of essential factors for obtaining final uniform BaTiO3 powder concerns uniform dispersion of BaCO3 powder and TiO2 powder in the early stage. Such technologies have been consistently developed.
For example, conventional technologies are disclosed in Korean Patent Application Publication Nos. 2002-0053749 and 2004-0038747. The Patent Application No. 2002-0053749 discloses barium titanate powder obtained by mixing barium compound and titanium dioxide having rutile ratio of up to 30% and BET specific surface area of at least 5 m2/g and calcining the same. Meanwhile, the Patent Application No. 2004-38747 teaches a technology of absorbing organic polymer compound into barium carbonate powder. According to inventions disclosed in the aforesaid patent application publications, advantageously, barium compound and titanium dioxide are mixed uniformly to enhance the degree of mixing. However despite dispersion of each element, the acicular shape of barium compounds remains unchanged, leading to inevitable contact among barium compounds due to their morphological properties. Consequently, there is a limit in obtaining optimal degree of mixing with titanium dioxide.
Another conventional technology is disclosed in Korean Patent Application Publication No. 2004-0020252. Herein, BaCO3 powder is dry-milled spherically, mixed with TiO2 powder, and then calcined. However according to the aforesaid technology, disadvantageously, such dry-milling does not reduce the number of BaCO3 particles, and high stress placed on BaCO3 does not disperse BaCO3 particles properly, thus leading to agglomeration. Large specific surface area of powder, or small particle size results in uniform dispersion, but BaCO3 according to the aforesaid technology does not diminish particle numbers, rendering uniform mixing with TiO2 difficult. Thus, BaTiO3 powder finally obtained agglomerates heavily among primary particles and forms secondary particles relatively bigger than primary particles, also causing uneven particle distribution of powder. BaTiO3 powder with such properties may be hardly dispersible when applied to the MLCC, and unsuitable for the dielectric ceramic use for up to 1 μm lamination to ensure a high-capacity capacitor.